Converter.cs

 protected override double Convert(double value, Type targetType, object parameter, CultureInfo culture)
{
    if (value > 0)
    {
        var result = value * (double)parameter;
        return result;
    }
    return value;
}

 <x:Double x:Key="HalfScreenButtonFontConverterValue">0.1</x:Double>

<Grid Grid.Row="2"
      ColumnDefinitions="*, *"
      ColumnSpacing="10"
      Padding="15"
      BackgroundColor="{StaticResource MainBackground}">

    <Button x:Name="LeftButton"
            FontSize="{
                Binding Source={x:Reference LeftButton}, 
                Path=Width, 
                Converter={StaticResource DimensChangeConverter},
                ConverterParameter={StaticResource HalfScreenButtonFontConverterValue}}" />

    <Button Grid.Column="1"
            FontSize="{
                Binding Source={x:Reference LeftButton}, 
                Path=Width, 
                Converter={StaticResource DimensChangeConverter},
                ConverterParameter={StaticResource HalfScreenButtonFontConverterValue}}" />
</Grid>